Metals and alloys are well known materials of construction for apparatus wherein aqueous systems may be concentrated, as by evaporation of water from the system. It is also well known that scale and other hydrophilic foulants deposit onto metal containing and alloy containing surfaces of such apparatus where aqueous systems such as saline water are subjected to evaporation and related processes in the apparatus. These deposits typically reduce heat transfer rates, resulting in substantial inefficiency of the processes such as increased heating media requirements and frequent cleanings.
Typically, the apparatus surfaces include such metals as copper, nickel, and alloys thereof. Alloys comprising copper and nickel are widely used in these surfaces. Such alloys are well known, e.g., Monel which consists essentially of 66 percent nickel, 31.5 percent copper, and minor amounts of manganese iron, carbon, sulfur, and silica.
Various method for removing salt from sea water have been developed over the years to relieve the scarcity of fresh water in many parts of the world. In addition, the ships at sea have been utilizing sea water as a source of drinking water for some time. One of the principal methods of purifying sea water is through the use of various types of evaporators such as flash evaporators, thin film distillation evaporators and submerged tube evaporators. Surfaces of many of these evaporators are formed of copper-nickel alloys.
Sea water contains a substantial amount of components which deposit onto metal containing and alloy containing surfaces as scale and related hydrophilic foulants. The alkaline scales, including calcium carbonate and magnesium hydroxide, are particularly troublesome in sea water evaporators. These scales form readily on surfaces of copper, nickel, and copper-nickel alloys.
In a number of methods heretofore proposed for decreasing deposition of scale from aqueous systems onto metalliferous surfaces of apparatus such as sea water distillation plants, chelating agents specific for chelating scale-forming metals included in the aqueous systems are added, as with the aqueous feed.
One of the methods proposed for inhibiting deposition of alkaline scale onto metal or alloy surfaces of sea water evaporators and related apparatus is treatment of the feed with sulfuric acid. This method has not proved entirely satisfactory in that it requires a high degree of technical sophistication on the part of the operators which may not be available in certain underdeveloped countries. In addition, handling and storing of large quantities of sulfuric acid poses some problems.
Addition of polyphophate-lignosulfonate mixtures has been proposed for inhibiting alkaline scale depositions, as for example in U.S. Pat. No. 2,782,162 to Lidell. However, these mixtures have not proved effective in saline water systems at high temperatures, addition thereof typically resulting in formation of calcium phosphate sludge at system temperatures above 195.degree.F.
Certain polyelectrolytes such as sodium polymethacrylate, while generally effective in preventing calcium carbonate scale at temperatures as high as 240.degree.F., have not proved entirely satisfactory in aqueous systems from which, when concentrated as in sea water evaporators, magnesium including foulants typically are deposited. For example, sodium polymethacrylate is generally ineffective for inhibiting magnesium hydroxide scale deposition from saline water concentrated to 70,000 parts per million total dissolved solids at system temperatures at which sea water distillation plants typically are operated.